1. Field of the Invention
The present invention relates to a tip-replaceable cutting tool in which a cutting insert is removably installed, in particular for cutting a configuration part.
2. Description of the Related Art
There is conventionally known a so-called screwed type of cutting tool in which a cutting insert having cutting edges is removably installed to a tool body by using a fastening screw such as a countersunk screw. This type of cutting tool is put into practical use in finish processing to a configuration part of a work piece. An example of the cutting tool used for cutting processing to the configuration part includes a chamfering cutter described in Japanese Patent Laid-Open No. 2005-28502.
This chamfering cutter is structured such that a rhombic plate-shaped cutting insert for chamfering cutter, in which a rhombic face is directed in a cutter rotational direction as a rake face and a cutting edge formed in a ridge portion of the rake face is inclined to be directed to a peripheral side as closer to a rear end side of the cutter body, is removably attached to an insert attachment seat formed in a front end portion of a cutter body.
In the cutting insert for a chamfering cutter, the cutting edges are formed in the ridge portions of the alternate rhombic face sides in a peripheral direction of the cutting insert out of the ridge portions in pairs of the rhombic faces and each peripheral face of the cutting insert connected to the cutting edges is formed as a flank and is inclined to gradually withdraw as it is away from the cutting edge.
In the chamfering cutter described in Japanese Patent Laid-Open No. 2005-28502, the cutting insert is fixed in a predetermined posture to the cutting body when a pair of peripheral faces extending from a sharp corner portion of the rhombic face contact wall faces of the insert attachment seat corresponding to the respective peripheral faces. Here, as viewed from a side opposing the bottom face of the insert attachment seat (or a rake face of the cutting insert), a fastening screw engaging to an inner peripheral face of an attachment hole of the cutting insert is deviated slightly closer to the two wall faces than the center of the attachment hole. Therefore the cutting insert is installed such that, by pressing the inner peripheral face of the attachment hole with the fastening screw, the pair of peripheral faces contacts the wall faces of the insert attachment seat corresponding to the respective peripheral faces.
However, at the time of screwing the fastening screw, the cutting insert is subjected to a rotational moment in a rotational direction of the fastening screw by contact between the fastening screw and the inner peripheral face of the attachment hole, and may be forcibly rotated in the rotational direction of the fastening screw in the insert fastening seat. Therefore since the cutting edge of the cutting insert can not be correctly positioned to the cutting body, the precision of a chamfered portion cut by this cutting edge is also deteriorated. Caused by a cutting resistance which the cutting insert receives at cutting processing, the cutting insert is forced to be rotated in a direction opposing the rotational direction. Therefore looseness of the fastening screw is generated, causing further rotation of the cutting insert, and in the worst case there is a possibility that breakage of the cutting edge is generated or the cutting insert falls down from the insert attachment seat.
The present invention is made in view of the foregoing problem, and an object of the present invention is to improve positioning precision of a cutting edge of a cutting insert to a tool body and enhance processing precision in a configuration part of a work piece.